**3. Structure**

The Earth has a thin silicate crust, which makes up 1% of the Earth's volume [5]. It is the uppermost top component of the lithosphere and floats on top of the upper mantle [6]. The crust plus the upper mantle is separated by the Mohorovicic discontinuity—a seismic and compositional boundary [6]. The crust varies in thickness as controlled by the law of isostasy according to Airy's model—the crust responds to topographical changes (loads or unloads) by changing its thickness as compensation, thus tending towards isostatic equilibrium [7]. The crust is thickest under mountain ranges and thinnest under mid-ocean ridges [6].

**3**

**4. Composition**

chemical composition.

*Introductory Chapter: Earth Crust - Origin, Structure, Composition and Evolution*

rise to temperature variations in the different crust types.

There are two main types of crust, the continental crust (underlie continents) and the oceanic crust (underlie ocean basins), the latter being denser and thinner but both being less dense than the mantle [6]. Approximately 35% of the Earth's crust is continental, while the other 65% is oceanic [8]. Continents are generally antipodic to oceans [9]. Conrad discontinuity, which lies at a depth of 5–20 km, separates the continental crust and oceanic crust [1]. Unlike continental crust, the oceanic crust has no granitic zone, but the mantle beneath the oceanic crust is possibly richer in radioactive elements than the mantle below the continental crust. The different locations of heat sources and thermal conductivity of the crust give

Within the continental crust, there are four layers—the upper, middle, lower and the lowest layers [10]. The first layer is mainly made of sedimentary rocks and volcanic rocks, and the P-wave velocities in this layer are less than 5.7 km/s [10]. The second layer is mainly made of granitic plutons and metamorphic rocks (lowgrade), and the P-wave velocities in this layer are between 5.7 and 6.4 km/s [11]. The third layer is mainly made of gabbroic cumulate, and the P-wave velocities in this layer range from 6.4 to 7.1 km/s [10]. The fourth layer is usually thin or missing [11],

The crust is carried as plates, which are slabs of lithosphere that carry oceanic crust, continental crust or both. They are carried by convection currents in the mantle, a process known as plate tectonics, which is driven by internal heat [1]. The plates meet at plate boundaries, which can be convergent boundaries, divergent boundaries or transform faults [1]. Interactions at plate boundaries, such as between crusts or between the crust and the mantle, can give rise to tectonic features such as oceanic ridges and volcanic arcs [6]. The crust undergoes physical and/or chemical changes in response to these interactions. For example, new oceanic crust develops at the opening rifts of divergent boundaries, forming mid-oceanic ridges through ridge push [12]. At convergent boundaries, dense oceanic crust subducts (slab pull) and produces magma due to the partial melting caused by the mantle's heat (thus creating a hotspot under the crust), which may rise and erupt, resulting in the formation of volcanic features, such as volcanoes, volcanic arcs and islands, on the non-subducted converging oceanic or continental crust [12]. This is seen in the Aleutian Islands. The colliding continental crust would result in the crust deforming into fold mountains [12], and an example of this is the Himalayan mountain range. Continental margins are long narrow belts [13] that form at the outer edges of major landmasses [14] that include continental and submarine mountain chains. These margins could be passive, active or transform. Passive margins are found between continental and oceanic crust and are tectonically inactive, thus having a smooth relief [14]. Active margins have more tectonic and seismic activity and have

and the P-wave velocities in this layer are between 7.1 and 7.6 km/s [10].

features such as volcanoes and high sediment availability [14].

crust that have since been consumed by subduction [15].

Another notable component of the Earth's structure is ophiolites. These are the fragments of oceanic crust and of the upper mantle that have undergone tectonic emplacement onto the continental crust [15]. They can be incorporated into both passive and active margins [16] and could be evidence of features of ancient oceanic

Minerals and rocks that make up the Earth's crust are the results of geological

activity, density and tectonic plate movement. Minerals have definite chemical composition, whereas rocks are made up of minerals and have no specific

*DOI: http://dx.doi.org/10.5772/intechopen.88100*

#### *Introductory Chapter: Earth Crust - Origin, Structure, Composition and Evolution DOI: http://dx.doi.org/10.5772/intechopen.88100*

There are two main types of crust, the continental crust (underlie continents) and the oceanic crust (underlie ocean basins), the latter being denser and thinner but both being less dense than the mantle [6]. Approximately 35% of the Earth's crust is continental, while the other 65% is oceanic [8]. Continents are generally antipodic to oceans [9]. Conrad discontinuity, which lies at a depth of 5–20 km, separates the continental crust and oceanic crust [1]. Unlike continental crust, the oceanic crust has no granitic zone, but the mantle beneath the oceanic crust is possibly richer in radioactive elements than the mantle below the continental crust. The different locations of heat sources and thermal conductivity of the crust give rise to temperature variations in the different crust types.

Within the continental crust, there are four layers—the upper, middle, lower and the lowest layers [10]. The first layer is mainly made of sedimentary rocks and volcanic rocks, and the P-wave velocities in this layer are less than 5.7 km/s [10]. The second layer is mainly made of granitic plutons and metamorphic rocks (lowgrade), and the P-wave velocities in this layer are between 5.7 and 6.4 km/s [11]. The third layer is mainly made of gabbroic cumulate, and the P-wave velocities in this layer range from 6.4 to 7.1 km/s [10]. The fourth layer is usually thin or missing [11], and the P-wave velocities in this layer are between 7.1 and 7.6 km/s [10].

The crust is carried as plates, which are slabs of lithosphere that carry oceanic crust, continental crust or both. They are carried by convection currents in the mantle, a process known as plate tectonics, which is driven by internal heat [1]. The plates meet at plate boundaries, which can be convergent boundaries, divergent boundaries or transform faults [1]. Interactions at plate boundaries, such as between crusts or between the crust and the mantle, can give rise to tectonic features such as oceanic ridges and volcanic arcs [6]. The crust undergoes physical and/or chemical changes in response to these interactions. For example, new oceanic crust develops at the opening rifts of divergent boundaries, forming mid-oceanic ridges through ridge push [12]. At convergent boundaries, dense oceanic crust subducts (slab pull) and produces magma due to the partial melting caused by the mantle's heat (thus creating a hotspot under the crust), which may rise and erupt, resulting in the formation of volcanic features, such as volcanoes, volcanic arcs and islands, on the non-subducted converging oceanic or continental crust [12]. This is seen in the Aleutian Islands. The colliding continental crust would result in the crust deforming into fold mountains [12], and an example of this is the Himalayan mountain range.

Continental margins are long narrow belts [13] that form at the outer edges of major landmasses [14] that include continental and submarine mountain chains. These margins could be passive, active or transform. Passive margins are found between continental and oceanic crust and are tectonically inactive, thus having a smooth relief [14]. Active margins have more tectonic and seismic activity and have features such as volcanoes and high sediment availability [14].

Another notable component of the Earth's structure is ophiolites. These are the fragments of oceanic crust and of the upper mantle that have undergone tectonic emplacement onto the continental crust [15]. They can be incorporated into both passive and active margins [16] and could be evidence of features of ancient oceanic crust that have since been consumed by subduction [15].
